Electrical capacitors made of thin film dielectric material which is metallized on both sides are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. U.S. Pat. No. 2,470,826 issued on May 24, 1949 to W. McMahon illustrates a capacitor in which a double sided metallized dielectric layer is folded an odd number of times in a longitudinal direction. The folded dielectric material may then be wound along its longitudinal dimension and end terminations may then be applied to it by spraying of metal against the edges of the wound capacitor segment. The type of capacitor described by McMahon patent is extremely desirable because substantially all of the dielectric layer of the capacitor is in the electric field and the capacitor may be made without the insertion of an additional dielectric layer in the main body of the capacitor, although a short initial unmetallized dielectric strip and a protective terminating dielectric strip may be employed with the capacitor, if desired.
The type of capacitor construction that is envisioned by McMahon patent has a number of decided manufacturing and electrical advantages. For example, the problem of masking the dielectric layer during electrode evaporation when the electrode areas are applied to the film is substantially reduced. In addition, no slitting is required as it is during the manufacture of conventional wound film capacitors. Furthermore, the capacitance per unit dielectric area is maximized because of the substantially full use of the dielectric layer in the electric field. Shorter winding lengths per unit capacitance are thereby required, reducing the winding labor. The dielectric lengths can also be calibrated and pre-cut to produce a capacitor of more accurate values. Another advantage of this type of capacitor is that there are no air layers between the metallized electrodes and the pleated film surfaces and this substantially increases the voltage, for example, one hundred volts or more, at which corona discharge starts, making this capacitor commercially attractive for new applications such as fluorescent lighting circuits. Previously, fifty gauge film thickness or more was required of wound film capacitors to withstand the corona. With the capacitor of the present invention, thirty-two gauge or less dielectric film thickness may be utilized, making wound film capacitors economically feasible for such applications.
The conventional metal electrode layer that is secured to the dielectric layer is aluminum. It has now been discovered that when one portion of an electrode is brought into contact with an adjacent portion of the same electrode there is a danger that the very thin aluminum oxide layer that occurs naturally on the aluminum electrode area can be augmented due to arcing when a high voltage pulse is applied to the electrode during test or use of the capacitor. When this occurs, the electrode area between the two adjacent electrode portions may be converted entirely to oxide, thereby opening one, or both, sections of the electrode with a consequent loss of capacitance and an increase in the probability of dielectric damage due to corona in A.C. applications. The present invention eliminates this problem by plating a very thin conductive layer of a metal that has a relatively low melting point to prevent dielectric degrading, is less readily oxidizable than aluminum and does not react adversely with the dielectric layer, on the aluminum layer before the aluminum oxide layer can form. The aluminum layer is generally on the order of 240 angstroms thick with a natural oxide layer of approximately 10 angstroms thick. Plating thickness of the protective layer is preferably at least 10 angstroms. The protective layer should be formed with a metal having an evaporation rate high enough to allow good plating economics. Moreover, the metal should have a relatively low heat of fusion, heat of vaporization and vaporization temperature. Metals that may be used for the protective layer are lead, tin and zinc, although other metals and alloys may be employed.
While the present invention is described by reference to a particular embodiment, it is apparent that the invention is useful for all types of metallized dielectric capacitors having dielectric layers that are metallized with aluminum wherein two electrode layers are adjacent each other. Examples of some capacitors which may utilize the present invention are described in copending applications Ser. No. 814,958 filed July 12, 1977, "Pleated Metallized Film Capacitor with Sprayed Edge Terminations"; Ser. No. 814,945 filed July 12, 1977, "Single Pleat Metallized Film Capacitor with Sprayed Edge Terminations"; Ser. No. 814,955 filed July 12, 1977, "Pleated Metallized Film Capacitor with Staggered Edges"; and Ser. No. 814,954 filed July 12, 1977, "Pleated Metallized Film Capacitor Wound About Its Center", all of which have been invented by the inventor of the present invention and all of which are assigned to the assignee of the present invention.